DNA is probably the most well-known molecule of biological origin, it is found in all living things on planet Earth. But… Why is DNA so important?
DNA (deoxyribonucleic acid) contains the instructions necessary for life: inside our DNA is encoded the information needed to make all the proteins in our body. Proteins fulfill a multitude of roles, determining the structure of cells and directing almost all metabolic processes in the body.
Differences in the genetic code are responsible for a multitude of phenomena observed in humans and animals: for example, why some people are more prone than others to developing certain diseases, or why dogs have glue, different eye colors or blood group. All our physical and psychological traits are determined by genetics, although the environment can then significantly influence our development.
We have all heard of DNA and we know its fundamental role in our body as a guardian of genetic information, but… Are there other functions? In this article we talk in depth about DNA, its structure and all its functions.
What exactly is DNA?
DNA is the acronym used for deoxyribonucleic acid. We can say that DNA is building block of all living beings, it contains all the genes necessary for the manufacture of proteins, molecules essential to the functioning of our body.
DNA contains our inherited material, which makes us what we are, no one has the same DNA as another: each person has a unique code contained in the long DNA molecule. The information in DNA is passed from parents to children and approximately half of a child’s DNA is of paternal origin and the other half maternal.
It is described in DNA as a polymer of nucleotides, i.e. a long chain made up of small molecules.
Nucleotides are the basic units of deoxyribonucleic acid (DNA). Each nucleotide can be divided into three parts: a carbohydrate (2-deoxyribose), a nitrogenous base and a phosphate group (derived from phosphoric acid).
Nucleotides are distinguished by their nitrogenous base, and it is the number of the base which is specified when presenting the DNA sequence, since the other two components are always the same. There are four different bases:
- Adenine (A)
- cytosine (C)
- Guanine (G)
- Thymine (T)
DNA takes the form of a double helix, when it comes to a three-dimensional level; it is made up of two chains linked by hydrogen bonds, forming a double-stranded molecule. The base pairs form the ladder-like spiral and the sugar phosphate backbone forms the supporting sides of the DNA helix.
The bases are aligned in sequential order along the chain, encoding the genetic information according to the criterion of complementarity: AT and GC. The size of adenine and guanine is greater than that of thymine and cytosine, which makes this criterion of complementarity necessary for the DNA to remain homogeneous.
On the other hand, DNA is found in the cell nucleus of eukaryotes, as well as in chloroplasts and mitochondria. In prokaryotic organisms, the molecule sits free in the cytoplasm in an irregularly shaped body called a nucleoid. Finally, it should be added that the structure of DNA differs between prokaryotic and eukaryotic cells. In eukaryotic cells, it has a linear structure and the ends of each chain are free; however, in prokaryotic cells, the DNA is contained in a long, circular double strand.
What is DNA used for?
DNA has three main functions in the body: store information (genes and complete genome), produce proteins (transcription and translation) and duplicate to ensure the transmission of information to daughter cells during cell division.
The information needed to build and maintain an organism is stored in DNA, which is passed from parent to child. The DNA that carries this information is called genomic DNA, and all of the genetic information is called the genome. We have more than two meters of DNA and our nuclei are much smaller: DNA is organized into compact molecules called chromatin, which correspond to the association of DNA, RNA and proteins. The chromatin is then assembled into chromosomes, highly organized structures that enable cell division.
The categories and parts of DNA
DNA can be divided into two broad categories: non-coding DNA and coding DNA. Let’s see their specific functions.
1. Coding DNA
You can’t talk about DNA coding without talking about genes. A gene is a section of DNA that influences a trait or characteristic of an organism., such as eye color or blood type. Genes have coding regions called open reading frames, as well as control sections called enhancers and promoters that influence the coding region to be transcribed. The total amount of information contained in the genome of an organism is called the genotype.
DNA has the information for the manufacture of proteins, which are called the workers of the organism, and which fulfill a multitude of functions; some proteins are structural, like hair or cartilage proteins, while others are functional, like enzymes.
The body uses 20 different amino acids to produce around 30,000 different proteins. The DNA molecule must tell the cell the order in which the amino acids should be joined.
Heredity determines which proteins will be produced, using DNA as a template to build them. Sometimes changes in the DNA code (mutations) cause proteins to malfunction, causing disease. However, on other occasions, code changes will cause beneficial changes in individuals, who can then better adapt to their surroundings.
A gene has DNA which is read and converted into a messenger RNA substance. This RNA transmits information between the DNA of the gene and the machinery responsible for making proteins. RNA acts as a blueprint for the production machinery so that amino acids are placed and connected in the correct order to produce a protein.
Although protein transcription is the fundamental role of DNA. The central dogma of DNA → RNA → protein biology has been shown to be wrong and, in fact, there are multiple processes that influence and transfer information. Some viruses use RNA as their source material (RNA viruses), and the process of passing information from RNA to DNA is known as reverse transcription or reverse transcription DNA. There are also non-coding RNA sequences that are created by transferring DNA sequences to RNA, and these can have a function without becoming proteins.
2. Non-coding DNA
About 90% of a person’s genome does not code for proteins. This part of DNA is called non-coding DNA. DNA can be conceptually divided into two categories, genes that code for proteins and non-genes. In many species, only a small part of the DNA codes for proteins – the exons – and these make up only around 1.5% of the human genome.
Non-coding DNA, also called junk or trash DNA, is DNA that does not code for a protein: sequences such as introns, viral recombinations, etc. Until recently, this DNA was thought to be useless until recent studies show that it is not. These sequences can regulate gene expression because they have affinity for proteins that can bind DNA and are called regulatory sequences.
Scientists have identified only a small percentage of the total existing regulatory sequences. The reason for the presence large amounts of non-coding DNA in eukaryotic genomes and differences in genome size between different species is still an enigma today. Although more and more functions of non-coding DNA are known, such as:
2.1. repetitive elements
Repetitive elements of a genome are also functional parts of a genome, account for more than half of the total nucleotides. A group of scientists from Yale University recently discovered a non-coding DNA sequence believed to play a role in allowing humans to develop the ability to use tools.
2.2. Telomeres and centromeres
Moreover, certain DNA sequences are responsible for the structure of chromosomes. Telomeres and centromeres contain few or no coding genes, but they are crucial in holding the chromosomal structure together.
2.3. DNA to RNA
Some genes do not code for proteins, but are transcribed into RNA molecules: ribosomal RNA, transfer RNA and interfering RNA (RNAi).
2.4. Alternative splicing
The arrangement of introns and exons in certain gene sequences is important because allows alternative splicing of pre-messenger RNA, creating different proteins from the same gene. Without this ability, the immune system would not exist.
Some non-coding DNA sequences are from genes lost during evolution. These pseudogenes can be useful because they can give rise to new genes with new functions.
2.6. Small sections of DNA
Other non-coding DNA sequences arise from the replication of small sections of DNA, which is also useful because tracking these repetitive sections of DNA can aid in phylogeny studies.
DNA is the molecule that contains hereditary information in humans; this information, contained in the DNA, allows the cell to know the order in which the amino acids which form the proteins must be assembled. Proteins are responsible for most of the body’s functions and a problem in their production can have major consequences for our health. However, when we speak of DNA → RNA → protein, we refer to the great dogma of biology and genes, forgetting 90% of DNA. Until recently, the role of DNA, which does not code for a protein, was considered irrelevant, but recent studies have uncovered more and more functions of these non-coding sequences called regulators.
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- Chatterjee N, et al. (2017). Mechanisms of DNA damage, repair and mutagenesis.
- Annunziato, A. (2008). DNA encapsidation: nucleosomes and chromatin. Nature Education, 1(1), 26.